Patent GB-687863-A (GB)
Title: Improvements in or relating to electric signalling systems
IPC: H04B14/06
Filed: 1950-05-02
Abstract: 687,863. Multiplex pulse code signalling. GENERAL ELECTRIC CO., Ltd., and STENNING, L. C. May 2, 1951 [May 2, 1950], No. 10852/50. Class 40 (v). In a pulse code modulation signalling system a first code group of pulses is produced approximately defining a signal amplitude sample and a second code group which represents the difference between the quantized amplitude obtained by decoding the first group and the actual amplitude sample. At the receiver, the two pulse groups are decoded in separate decoders, the outputs of which are combined. In the embodiment described, a a twelve-channel multiplex system has coding and decoding arrangements common to all channels, the channel sample recurrence frequency being 8 Kc/s. and the channel periods being of 10.4 microseconds duration. The distributer 1, Fig. 1, provides contiguous channel sample amplitudes, graph PAM, Fig. 4, from the channels Ch1, Ch2, Ch3, &c., which, after amplification in circuit 2, are reduced to one-sixteenth of their amplitude in attenuator 3 and applied to condenser Ca, through electronic switch S1. This switch is closed by pulses 28, graph S1, and between the occurrence of these pulses the sample stored on condenser Ca, graph Ca, is converted to a five-unit code, graph PC1, in the coarse coder 4. The fine coder 6 is supplied as follows. The sample amplitudes from amplifier 2 are stored alternately on condensers Cb1, Cb2, see graphs Cb1, Cb2, by the operation of electronic switches S2a, S2b, by the pulses of graphs S2a, S2b, respectively. Thereafter the appropriate one of electronic switches S3a, S3b is operated, by the pulses of graphs S3a or S3b, to share the charge of condenser Cb1 or Cb2 with condenser Cc. However, by the operation of electronic switch S5, by the pulses 30 of graph S5, condenser Cc has previously been charged with the opposite polarity from the quantized output of a decoder 5 connected to the output of the coarse coder 4. Hence the difference voltage to be applied to the fine coder 6 is produced across condensers Cc and Cb1 or Cb2, levels 44 and 45, for example, on graphs Cb1, Cc, respectively. The operation of switch S4, by the pulses of graph S4, charges a condenser Cd from condenser Cc to provide the input to the coder 6, graph Cd, which produces the three-unit code signal, graph PC2. In order to compress the two pulse code groups so that together they occupy only the time allotted to one channel period, the outputs of coders 4, 6 are applied, after shortening the pulses in gating circuits, to tapped delay lines 7, 8 respectively, the tappings from which are connected to the inputs of an electronic distributer S6, the cycle frequency of which is equal to the channel recurrence frequency. Circuits for performing the operations of Fig. 1, are described, Figs. 2, 3 (not shown). At the receiver, Fig. 6, the two compressed pulse groups for each channel sample are selected and separated by distributer S10 and applied for re-expansion to delay networks 101, 102, respectively, connected to distributers S11, S12 respectively, feeding the coarse and fine decoders 105, 106 respectively. By an arrangement of switches S13a, S13b, S15a, S15b, and condensers Cfl, Cf2, similar to those at the output of amplifier 2, Fig. 1, the decoded coarse channel samples are fed in succession to condenser Cg. The fine samples are added thereto by operation of switch S14 and the reconstituted channel samples passed to the channel distributer 127.